Optical member

ABSTRACT

There is provided an optical member capable of causing, when used for a liquid crystal display apparatus, light output from a light source to enter a liquid crystal display panel with high efficiency. An optical member according to an embodiment of the present invention includes: a polarizing plate; a reflective polarizer; a polarization converting layer; and a prism layer, the polarizing plate, the reflective polarizer, the polarization converting layer, and the prism layer being integrated in the stated order, wherein the polarization converting layer satisfies the following expression: L 90 /L 0 ≧0.2.

BACKGROUND OF THE INVENTION

This application claims priority under 35 U.S.C Section 119 to JapanesePatent Application No. 2016-171604 filed on Sep. 2, 2016 which areherein incorporated by reference.

1. Field of the Invention

The present invention relates to an optical member.

2. Description of the Related Art

In recent years, as a display, a liquid crystal display apparatus usinga surface light source device has been remarkably widespread. In aliquid crystal display apparatus including an edge light-type surfacelight source device, for example, light emitted from a light sourceenters a light guide plate, and propagates through an inside of thelight guide plate while repeating total reflection on a light outputsurface (liquid crystal cell-side surface) of the light guide plate anda back surface thereof. Part of the light that propagates through theinside of the light guide plate allows a traveling direction thereof tobe changed by a light scattering body or the like, which is arranged onthe back surface of the light guide plate or the like, and is outputfrom the light output surface to an outside of the light guide plate.Such light output from the light output surface of the light guide plateis diffused and condensed by various optical sheets, such as a diffusingsheet, a prism sheet, and a luminance enhancement film, and thereafter,the light enters a liquid crystal display panel in which polarizingplates are arranged on both sides of a liquid crystal cell. Liquidcrystal molecules of a liquid crystal layer of the liquid crystal cellare driven for each of pixels to control transmission and absorption ofthe incident light. As a result, an image is displayed.

Typically, the above-mentioned prism sheet is fitted into a casing ofthe surface light source device, and is arranged close to the lightoutput surface of the light guide plate. In a liquid crystal displayapparatus using such a surface light source device as described above,the prism sheet and the light guide plate are rubbed against each otherwhen installing the prism sheet or under an actual usage environment,and the light guide plate is flawed in some cases. In order to solvesuch a problem, a technology for integrating the prism sheet with alight source-side polarizing plate is proposed (Japanese PatentApplication Laid-open No. Hei 11-295714).

SUMMARY OF THE INVENTION

However, a liquid crystal display apparatus using the polarizing plateintegrated with the prism sheet as described above has a problem ofbeing dark because of its insufficient front luminance. The presentinvention has been made in view of the problem, and an object of thepresent invention is to provide an optical member capable of causing,when used for a liquid crystal display apparatus, light output from alight source to enter a liquid crystal display panel with highefficiency.

An optical member according to an embodiment of the present inventionincludes: a polarizing plate; a reflective polarizer; a polarizationconverting layer; and a prism layer, the polarizing plate, thereflective polarizer, the polarization converting laver, and the prismlayer being integrated in the stated order, wherein the polarizationconverting layer satisfies the following expression: L₉₀/L₀≧0.2 where,with regard to a luminance of natural light transmitted through a firstpolarizer and a second polarizer each having a polarization degree of99.99%, L₀ represents a parallel luminance, which is the luminance whenthe polarization converting layer is arranged between the firstpolarizer and the second polarizer arranged so that absorption axesthereof are parallel to each other, and L₉₀ represents a perpendicularluminance, which is the luminance when the polarization converting layeris arranged between the first polarizer and the second polarizerarranged so that the absorption axes thereof are perpendicular to eachother.

In one embodiment of the present invention, the polarization convertinglayer includes a retardation layer, and the retardation layer has anin-plane retardation Re (550) of 3,500 nm or more, where Re(550)represents an in-plane retardation measured with light having awavelength of 550 nm at 23° C.

In one embodiment of the present invention, the polarization convertinglayer includes a light diffusing layer, and the light diffusing layerhas a haze value of from 80% to 99.9%.

In one embodiment of the present invention, the polarization convertinglayer includes a λ/4 plate, wherein the λ/4 plate has an in-planeretardation Re(550) of from 80 nm to 200 nm, and an angle formed betweena slow axis of the λ/4 plate and a reflection axis of the reflectivepolarizer is from 30° to 60°.

In one embodiment of the present invention, the optical member furtherincludes a low-refractive index layer integrated at one of a positionbetween the reflective polarizer and the prism layer, and a position onan opposite side of the reflective polarizer to the prism layer, whereinthe low-refractive index layer has a refractive index of 1.30 or less.

In one embodiment of the present invention, the reflective polarizerincludes a linearly polarized light separation-type reflectivepolarizer.

According to the present invention, the optical member capable ofcausing light output from a light source to enter a liquid crystaldisplay panel with high efficiency can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an optical member according to oneembodiment of the present invention.

FIG. 2 is a cross-sectional view of an optical member according toanother embodiment of the present invention.

FIG. 3 is a cross-sectional view of an optical member according to stillanother embodiment of the present invention.

FIG. 4 is a cross-sectional view of an optical member according to vetstill another embodiment of the present invention.

FIG. 5 is a cross-sectional view of an optical member according to evenyet still another embodiment of the present invention.

FIG. 6 is a schematic perspective view of an example of a reflectivepolarizer that may be used for the optical member of the presentinvention.

FIG. 7 is a schematic cross-sectional view of a liquid crystal displayapparatus according to one embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention are described below. However, thepresent invention is not limited to these embodiments.

Definitions of Terms and Symbols

The definitions of terms and symbols used herein are as follows.

(1) Refractive Indices (nx, ny, and nz)

A symbol “nx” represents a refractive index in a direction in which anin-plane refractive index is maximum (that is, slow axis direction),“lay” represents a refractive index in a direction perpendicular to theslow axis in the plane (that is, fast axis direction), and “nz”represents a refractive index in a thickness direction.

(2) In-Plane Retardation (Re)

The term “Re (λ)” refers to the in-plane retardation of a film measuredat 23° C. with light having a wavelength of λ nm. The Re (λ) isdetermined from the equation “Re (λ)=(nx−ny)×d” when the thickness ofthe film is represented by d (nm). For example, the term “Re(550)”refers to the in-plane retardation of the film measured at 23° C. withlight having a wavelength of 550 nm.

(3) Thickness Direction Retardation (Rth)

The term “Rth (λ)” refers to the thickness direction retardation of thefilm measured at 23° C. with light having a wavelength of λ nm. Forexample, the term “Rth (550)” refers to the thickness directionretardation of the film measured at 23° C. with light having awavelength of 550 nm. The Rth (λ) is determined from the equation “Rth(λ)=(nx−nz)×d” when the thickness of the film is represented by d (nm).

(4) Nz Coefficient

An Nz coefficient is determined from the equation “Nz=Rth/Re”.

A. Entire Configuration of Optical Member

An optical member includes a polarizing plate 10, a reflective polarizer20, a polarization converting layer 30, and a prism sheet 40 in thestated order, in which the polarizing plate 10, the reflective polarizer20, the polarization converting layer 30, and the prism sheet 40 areintegrated. The prism sheet 40 typically includes a substrate portion 41and a prism portion 42 (prism layer).

The polarization converting layer 30 is configured to convert (ordepolarize) the polarization state of light entering from the prismsheet 40 side and output the light to the reflective polarizer 20 side.The polarization converting layer 30 has a polarization conversiondegree P_(CON), which is expressed by the following equation, of 0.2 ormore:

Polarization conversion degree (P_(CON))=L₉₀/L₀

where, with regard to the luminance of natural light of visible lighttransmitted through a first polarizer and a second polarizer each havinga polarization degree of 99.99%, L₀ (parallel luminance) represents theluminance when the polarization converting layer is arranged between thefirst polarizer and the second polarizer arranged so that the absorptionaxes thereof are parallel to each other, and L₉₀ (perpendicularluminance) represents the luminance when the polarization convertinglayer is arranged between the first polarizer and the second polarizerarranged so that the absorption axes thereof are perpendicular to eachother. According to the above-mentioned configuration, when the opticalmember is arranged between a polarizing plate on the opposite side of aliquid crystal display panel to a viewer side and a backlight unit,light output from the backlight unit and having its polarization stateconverted (or depolarized) by the polarization converting layer 30enters the reflective polarizer 20. With this, the utilizationefficiency of the light output from the backlight unit can be enhanced.

The optical member may include any of various optical sheets in additionto the polarizing plate 10, the reflective polarizer 20, thepolarization converting layer 30, and the prism sheet 40.

FIG. 1 is a cross-sectional view of an optical member 100 according toone embodiment of the present invention. FIG. 2 is a cross-sectionalview of an optical member according to another embodiment of the presentinvention. An optical member 101 illustrated in FIG. 2 includes thepolarizing plate 10, a low-refractive index layer 60, a light diffusinglayer 50, the reflective polarizer 20, the polarization converting layer30, and the prism sheet 40 in the stated order. FIG. 3 is across-sectional view of an optical member according to still anotherembodiment of the present invention. An optical member 102 illustratedin FIG. 3 includes the polarizing plate 10, the low-refractive indexlayer 60, the reflective polarizer 20, the polarization converting layer30, the light diffusing layer 50, and the prism sheet 40 in the statedorder. FIG. 4 is a cross-sectional view of an optical member accordingto yet still another embodiment of the present invention. An opticalmember 103 illustrated in FIG. 4 includes the polarizing plate 10, thelight diffusing layer 50, the reflective polarizer 20, the polarizationconverting layer 30, the low-refractive index layer 60, and the prismsheet 40 in the stated order. FIG. 5 is a cross-sectional view of anoptical member according to even yet still another embodiment of thepresent invention. An optical member 104 illustrated in FIG. 5 includesthe polarizing plate 10, the light diffusing layer 50, the reflectivepolarizer 20, the low-refractive index layer 60, and the prism sheet 40in the stated order, in which the prism sheet 40 includes thepolarization converting layer 30 configured to function as the substrateportion, and the prism portion 42 (prism layer). Two or more of theabove-mentioned embodiments may be combined.

B. Polarizing Plate

The polarizing plate 10 typically includes a polarizer 11, a protectivelayer 12 arranged on one side of the polarizer 11, and a protectivelayer 13 arranged on the other side of the polarizer 11. The polarizeris typically an absorption-type polarizer.

B-1. Polarizer

Any appropriate polarizer may be adopted as the absorption-typepolarizer depending on purposes. For example, a resin film for formingthe polarizer may be a single-layer resin film, or may be a laminate oftwo or more layers.

Specific examples of the polarizer including a single-layer resin filminclude: a polarizer obtained by subjecting a hydrophilic polymer film,such as a polyvinyl alcohol (PVA)-based film, a partially formalizedPVA-based film, or an ethylene-vinyl acetate copolymer-based partiallysaponified film, to dyeing treatment with a dichromatic substance, suchas iodine or a dichromatic dye, and stretching treatment; and apolyene-based alignment film, such as a dehydration-treated product ofPVA or a dehydrochlorination-treated product of polyvinyl chloride.polarizer obtained by dyeing the PVA-based film with iodine anduniaxially stretching the resultant is preferably used because thepolarizer is excellent in optical characteristics.

The dyeing with iodine is performed by, for example, immersing thePVA-based film in an aqueous solution of iodine. The stretching ratio ofthe uniaxial stretching is preferably from 3 times to 7 times. Thestretching may be performed after the dyeing treatment, or may beperformed while the dyeing is performed. In addition, the dyeing may beperformed after the stretching has been performed. The PVA-based film issubjected to swelling treatment, cross-linking treatment, washingtreatment, drying treatment, or the like as required. For example, whenthe PVA-based film is immersed in water to be washed with water beforethe dyeing, contamination or an antiblocking agent on the surface of thePVA-based film can be washed off. In addition, the PVA-based film isswollen and hence dyeing unevenness or the like can be prevented.

The polarizer obtained by using the laminate is specifically, forexample, a polarizer obtained by using a laminate of a resin substrateand a PVA-based resin layer (PVA-based resin film) laminated on theresin substrate, or a laminate of a resin substrate and a PVA-basedresin layer formed on the resin substrate through application. Thepolarizer obtained by using the laminate of the resin substrate and thePVA-based resin layer formed on the resin substrate through applicationmay be produced by, for example, a method involving: applying aPVA-based resin solution to the resin substrate; drying the solution toform the PVA-based resin layer on the resin substrate, thereby providingthe laminate of the resin substrate and the PVA-based resin layer; andstretching and dyeing the laminate to turn the PVA-based resin layerinto the polarizer. In this embodiment, the stretching typicallyincludes the stretching of the laminate under a state in which thelaminate is immersed in an aqueous solution of boric acid. Thestretching may further include the aerial stretching of the laminate athigh temperature (e.g., 95° C. or more) before the stretching in theaqueous solution of boric acid as required. The resultant laminate ofthe resin substrate and the polarizer may be used as it is (i.e., theresin substrate may be used as a protective layer for the polarizer).Alternatively, a product obtained as described below may be used: theresin substrate is peeled from the laminate of the resin substrate andthe polarizer, and any appropriate protective layer in accordance withpurposes is laminated on the peeling surface. Details of such method ofproducing a polarizer are disclosed in, for example, Japanese PatentApplication Laid-open No. 2012-73580. The entire disclosure of thelaid-open publication is incorporated herein by reference.

The thickness of the polarizer is typically from 1 μm to 80 μm. Theupper limit of the thickness of the polarizer is preferably 50 μm, morepreferably 25 μm, particularly preferably 12 μm. The lower limit of thethickness of the polarizer is preferably 1 μm, more preferably 3 μm.When the thickness of the polarizer falls within such range, curling atthe time of heating can be satisfactorily suppressed, and besides,satisfactory external appearance durability at the time of heating isobtained.

The transmittance of the polarizer (also referred to as “single layertransmittance”) at a wavelength of 589 nm is preferably 41% or more,more preferably 42% or more. The theoretical upper limit of the singlelayer transmittance is 50%. In addition, the polarization degree thereofis preferably from 99.5% to 100%, more preferably from 99.9% to 100%. Aslong as the polarization degree falls within the range, contrast in afront direction can be made higher when the polarizer is used in aliquid crystal display apparatus.

The single layer transmittance and polarization degree described abovecan be measured with a spectrophotometer. A specific measurement methodfor the polarization degree described above may involve measuring theparallel transmittance (H₀) and perpendicular transmittance (H₉₀) of thepolarizer, and determining the polarization degree through the followingexpression: polarization degree (%)={(H₀−H₉₀)/(H₀+H₉₀)}^(1/2)×100. Theparallel transmittance (H₀) described above refers to a value of atransmittance of a parallel-type laminated polarizer manufactured bycausing two identical polarizers to overlap with each other so thatabsorption axes thereof are parallel to each other. In addition, theperpendicular transmittance (H₉₀) described above refers to a value of atransmittance of a perpendicular-type laminated polarizer manufacturedby causing two identical polarizers to overlap with each other so thatabsorption axes thereof are perpendicular to each other. Each of thosetransmittances is a Y value obtained through visibility correction withthe two-degree field of view (C light source) of JIS Z 8701-1982.

The polarizing plate 10 is typically provided in a long shape (e.g., aroll shape) and used in the production of an optical member. In oneembodiment, the polarizer has an absorption axis in its lengthwisedirection. Such polarizer can be obtained by a production method thathas been conventionally employed in the industry (e.g., such productionmethod as described above). In another embodiment, the polarizer has theabsorption axis in its widthwise direction. The optical member of thepresent invention can be produced by laminating such polarizer togetherwith a linearly polarized light separation-type reflective polarizerhaving a reflection axis in its widthwise direction according to theso-called roll-to-roll process, and hence the efficiency of theproduction can be significantly improved.

B-2. Protective Layer

The protective layer is formed of any appropriate film that may be usedas a protective film for the polarizing plate. Specific examples of amaterial serving as a main component of the film include transparentresins, such as a cellulose-based resin, such as triacetylcellulose(TAC), a polyester-based resin, a polyvinyl alcohol-based resin, apolycarbonate-based resin, a polyamide-based resin, a polyimide-basedresin, a polyether sulfone-based resin, a polysulfone-based resin, apolystyrene-based resin, a polynorbornene-based resin, apolyolefin-based resin, a (meth) acrylic resin, and an acetate-basedresin. Another example thereof is a thermosetting resin or a UV-curableresin, such as a (meth)acrylic resin, a urethane-based resin, a (meth)acrylic urethane-based resin, an epoxy-based resin, or a silicone-basedresin. Still another example thereof is a glassy polymer, such as asiloxane-based polymer. Further, a polymer film disclosed in JapanesePatent Application Laid-open No. 2001-343529 (WO 01/37007 A1) may alsobe used. As a material for the film, for example, there may be used aresin composition containing: a thermoplastic resin having a substitutedor unsubstituted imide group in a side chain; and a thermoplastic resinhaving a substituted or unsubstituted phenyl group and a nitrile groupin side chains. An example thereof is a resin composition containing analternate copolymer formed of isobutene and N-methylmaleimide, and anacrylonitrile-styrene copolymer. The polymer film may be an extrudedproduct of the resin composition, for example. The protective layers maybe identical to or different from each other.

The thickness of each of the protective layers is preferably from 10 μmto 100 μm, more preferably from 20 μm to 100 μm. Each of the protectivelayers may be laminated on the polarizer through intermediation of anadhesion layer (specifically an adhesive layer or a pressure-sensitiveadhesive layer), or may be laminated so as to be in close contact withthe polarizer (without through the adhesion layer). The adhesive layeris formed of any appropriate adhesive. The adhesive is, for example, awater-soluble adhesive using a polyvinyl alcohol-based resin as a maincomponent. The water-soluble adhesive using the polyvinyl alcohol-basedresin as a main component can preferably further contain a metalcompound colloid. The metal compound colloid can be such that metalcompound fine particles are dispersed in a dispersion medium, and thecolloid can be a colloid that electrostatically stabilizes as a resultof interactive repulsion between the charges of the same kind of thefine particles to permanently have stability. The average particlediameter of the fine particles forming the metal compound colloid may beany appropriate value as long as the average particle diameter does notadversely affect the optical characteristics of the polarizer, such as apolarization characteristic. The average particle diameter is preferablyfrom 1 nm to 100 nm, more preferably from 1 nm to 50 nm. This is becausethe fine particles can be uniformly dispersed in the adhesive layer, itsadhesion can be secured, and a knick can be suppressed. The term “knick”refers to a local uneven defect that occurs at an interface between thepolarizer and each of the protective layers.

C. Reflective Polarizer

The reflective polarizer 20 has a function of transmitting polarizedlight in a specific polarization state (polarization direction) andreflecting light in a polarization state other than the foregoing. Thereflective polarizer 20 may be of a linearly polarized light separationtype or may be of a circularly polarized light separation type.Description is hereinafter given by taking the linearly polarized lightseparation-type reflective polarizer as an example. The circularlypolarized light separation-type reflective polarizer is, for example, alaminate of a film obtained by fixing a cholesteric liquid crystal and aλ/4 plate.

FIG. 6 is a schematic perspective view of an example of a reflectivepolarizer. The reflective polarizer is a multilayer laminate obtained byalternately laminating a layer A having birefringence and a layer Bsubstantially free of birefringence. For example, the total number ofthe layers of such multilayer laminate can be from 50 to 1,000. In theillustrated example, a refractive index nx in the x-axis direction ofthe A layer is larger than a refractive index ny in the y-axis directionthereof, and a refractive index nx in the x-axis direction of the Blayer and a refractive index ny in the y-axis direction thereof aresubstantially equal to each other. Therefore, a refractive indexdifference between the A layer and the B layer is large in the x-axisdirection, and is substantially zero in the v-axis direction. As aresult, the x-axis direction serves as a reflection axis and the y-axisdirection serves as a transmission axis. The refractive index differencebetween the A layer and the B layer in the x-axis direction ispreferably from 0.2 to 0.3. The x-axis direction corresponds to thestretching direction of the reflective polarizer in a production methodto be described later.

The A layer is preferably formed of a material that expressesbirefringence when stretched. Typical examples of such material includenaphthalene dicarboxylic acid polyester (e.g., polyethylenenaphthalate), polycarbonate, and an acrylic resin (e.g., polymethylmethacrylate). Of those, the polyethylene naphthalate is preferred. TheB layer is preferably formed of a material that is substantially free ofexpressing birefringence even when stretched. Such material istypically, for example, a copolyester of naphthalene dicarboxylic acidand terephthalic acid.

The reflective polarizer transmits light having a first polarizationdirection (e.g., a p-wave) and reflects light having a secondpolarization direction perpendicular to the first polarization direction(e.g., a s-wave) at an interface between the A layer and the B layer.Part of the reflected light passes as light having the firstpolarization direction through the interface between the A layer and theB layer, and the other part thereof is reflected as light having thesecond polarization direction at the interface. Such reflection andtransmission are repeated many times in the reflective polarizer, andhence the utilization efficiency of light can be improved.

In one embodiment, the reflective polarizer may include a reflectivelayer R as the outermost layer on an opposite side to the polarizingplate 10 as illustrated in FIG. 6. Light that has finally returned tothe outermost portion of the reflective polarizer without being utilizedcan be further utilized by arranging the reflective layer R, and hencethe utilization efficiency of the light can be further improved. Thereflective layer R typically expresses a reflecting function by virtueof the multilayer structure of a polyester resin layer.

The total thickness of the reflective polarizer may be appropriately setdepending on, for example, purposes and the total number of the layersin the reflective polarizer. The total thickness of the reflectivepolarizer is preferably from 10 μm to 150 μm. When the total thicknessfalls within such range, a distance between the light diffusing layerand the prism portion of the prism sheet can be caused to fall within adesired range. As a result, a liquid crystal display apparatus thatsuppresses the occurrence of the moire and has high luminance can beachieved.

In one embodiment, in the optical member, the reflective polarizer 20 isarranged so as to transmit light having a polarization directionparallel to the transmission axis of the polarizing plate 10(substantially the polarizer 11). That is, the reflective polarizer 20is arranged so that its transmission axis is in a directionapproximately parallel to the direction of the transmission axis of thepolarizing plate 10. With such configuration, light to be absorbed bythe polarizing plate 10 can be recycled, the utilization efficiency canbe further improved, and the luminance can be improved.

The reflective polarizer can be typically produced by combiningco-extrusion and lateral stretching. The co-extrusion may be performedby any appropriate system. For example, the system may be a feed blocksystem or may be a multi-manifold system. For example, a material forforming the A layer and a material for forming the B layer are extrudedin a feed block, and are then formed into a plurality of layers with amultiplier. Such apparatus for forming the materials into a plurality oflayers is known to one skilled in the art. Next, the resultant longmultilayer laminate is typically stretched in a direction (TD)perpendicular to its conveying direction. The material for forming the Alayer (e.g., polyethylene naphthalate) is increased in refractive indexonly in the stretching direction by the lateral stretching, and as aresult, expresses birefringence. The material for forming the B layer(e.g., the copolyester of naphthalene dicarboxylic acid and terephthalicacid) is not increased in refractive index in any direction even by thelateral stretching. As a result, a reflective polarizer having areflection axis in the stretching direction (TD) and having atransmission axis in the conveying direction (MD) can be obtained (theTD corresponds to the x-axis direction of FIG. 6 and the MD correspondsto the y-axis direction thereof). A stretching operation may beperformed with any appropriate apparatus.

A polarizer disclosed in, for example, Japanese Patent TranslationPublication No. Hei 9-507308 may be used as the reflective polarizer.

A commercial product may be used as it is as the reflective polarizer,or the commercial product may be subjected to secondary processing(e.g., stretching) before use. The commercial product is, for example, aproduct available under the product name “APCF” from Nitto DenkoCorporation, a product available under the product name “DBEF” from 3MCompany, or a product available under the product name “APF” from 3MCompany.

The reflective polarizer 20 is bonded to an adjacent layer throughintermediation of any appropriate adhesion layer (e.g., an adhesivelayer or a pressure-sensitive adhesive layer: not shown). When thereflective polarizer 20 is adjacent to the light diffusing layer 50 asillustrated in FIG. 2, FIG. 4, and FIG. 5 and the light diffusing layer50 is formed of a light diffusing pressure-sensitive adhesive, theadhesion layer between the reflective polarizer 20 and the lightdiffusing layer 50 may be absent.

D. Polarization Converting Layer

As described above, the polarization converting layer 30 is configuredto convert the polarization state of light entering from the prism sheet40 side and output the light to the reflective polarizer 20 side. In oneembodiment, the polarization converting layer 30 is arranged between thereflective polarizer 20 and the prism sheet 40. In another embodiment,the polarization converting layer 30 is configured to function as thesubstrate portion of the prism sheet, and is arranged between thereflective polarizer 20 and the prism portion 42.

As described above, the polarization converting layer 30 has apolarization conversion degree P_(CON), which is expressed by thefollowing equation, of 0.2 or more.

Polarization conversion degree (P_(CON))=L₉₀/L₀

The polarization conversion degree P_(CON) of the polarizationconverting layer 30 is preferably 0.3 or more, more preferably 0.5 ormore, still more preferably 5 or more. Such polarization convertinglayer may include, for example, a high-retardation layer, a lightdiffusing layer, or a λ/4 plate.

D-1. High-Retardation Layer

The high-retardation layer is formed of a transparent material havingbirefringence. The in-plane retardation Re (550) of the high-retardationlayer is preferably 3,500 nm or more, more preferably 4,000 nm or more.The thickness of the high-retardation layer is preferably from 0.1 μm to200 μm, more preferably from 1 μm to 100 μm. An angle formed between theslow axis of the high-retardation layer and the reflection axis of thereflective polarizer 20 is preferably from 30° to 60°, more preferablyfrom 40° to 50°, still more preferably about 45°.

The birefringence Δn_(xy) of the high-retardation layer is, for example,0.1 or more, preferably 0.2 or more. Meanwhile, the upper limit of thebirefringence Δn_(xy) is, for example, 1, preferably 0.8. When thebirefringence is optimized to such range, a high-retardation layer thatis thin and has desired optical characteristics can be obtained. The Nzcoefficient of the high-retardation layer is preferably from 0.9 to 3,more preferably from 0.9 to 2.5, still more preferably from 0.9 to 1.5,particularly preferably from 0.9 to 1.3.

Any appropriate material may be adopted as the material for forming thehigh-retardation layer. Examples of the material include polyesters,such as polyethylene terephthalate and polyethylene naphthalate,polycarbonate, polystyrene, polyetheretherketone, polyphenylene sulfide,and a cycloolefin polymer. In particular, polyesters typified bypolyethylene terephthalate each have large intrinsic birefringence andprovide a large in-plane retardation relatively easily even when havinga small thickness, and hence may be suitably used. The high-retardationlayer is typically formed of polyethylene terephthalate.

D-2. Light Diffusing Layer

The light diffusing layer includes, for example, a diffusing sheet. Thehaze value of the diffusing sheet is preferably from 80% to 99.9%, morepreferably from 90% to 99.9%. The thickness of the diffusing sheet maybe appropriately adjusted depending on, for example, its configurationand diffusing performance, and is preferably from 1 μm to 200 μm, morepreferably from 1 μm to 100 μm.

The diffusing sheet typically includes a light diffusing element. Thelight diffusing element includes a matrix and light-diffusible fineparticles dispersed in the matrix. The matrix is formed of, for example,an acrylic resin, an aliphatic (e.g., polyolefin) resin, or aurethane-based resin. The light-diffusible fine particles are preferablypolymer fine particles. As a material for the polymer fine particles,there are given, for example, a silicone resin, a methacrylic resin(e.g., polymethyl methacrylate), a polystyrene resin, a polyurethaneresin, and a melamine resin. Those resins each have excellentdispersibility in a pressure-sensitive adhesive and an appropriaterefractive index difference from the pressure-sensitive adhesive, andhence allow a diffusing sheet excellent in diffusing performance to beobtained. Of those, a silicone resin or polymethyl methacrylate ispreferred. The shape of each of the light-diffusible fine particles maybe, for example, a true spherical shape, a flat shape, or an amorphousshape. The light-diffusible fine particles may be used alone or incombination thereof.

The volume-average particle diameter of the light-diffusible fineparticles is preferably from 0.1 μm to 50 μm, more preferably from 1 μmto 10 μm. When the volume-average particle diameter is set to fallwithin the above-mentioned range, a diffusing sheet having excellentlight diffusing performance can be obtained. The volume-average particlediameter may be measured with, for example, an ultracentrifugalautomatic particle size distribution-measuring apparatus.

The refractive index of each of the light-diffusible fine particles ispreferably from 1.30 to 2.80, more preferably from 1.4 to 1.8.

The absolute value of a refractive index difference between each of thelight-diffusible fine particles and the matrix is preferably more than 0and 1 or less, more preferably more than 0 and 0.8 or less.

The content of the light-diffusible fine particles in the diffusingsheet is preferably 1 wt % or more and less than 100 wt %, morepreferably from 5 wt % to 50 wt %. When the content of thelight-diffusible fine particles is set to fall within theabove-mentioned range, a diffusing sheet having excellent lightdiffusing performance can be obtained.

The diffusing sheet may contain any appropriate additive. Examples ofthe additive include an antistatic agent and an antioxidant.

D-3. λ/4 Plate

The thickness of the λ/4 plate is preferably from 0.1 μm to 200 μm, morepreferably from 0.5 μm to 100 μm. The λ/4 plate typically has a slowaxis. An angle formed between the slow axis of the λ/4 plate and thereflection axis of the reflective polarizer 20 is preferably from 30° to60°, more preferably from 40° to 50°, still more preferably about 45°.The in-plane retardation Re (550) of the λ/4 plate is preferably from 70nm to 200 nm, more preferably from 75 nm to 180 nm, still morepreferably from 80 nm to 160 nm. The λ/4 plate preferably has arefractive index characteristic of showing a relationship of nx>ny≧nz.Herein, “ny=nz” encompasses not only a case in which ny and nz areexactly equal to each other, but also a case in which ny and nz aresubstantially equal to each other. Therefore, a relationship of ny<nzmay be satisfied without impairing the effect of the present invention.

The birefringence Δn_(xy) of the λ/4 plate is, for example, 0.0025 ormore, preferably 0.0028 or more. Meanwhile, the upper limit of thebirefringence Δn_(xy) is, for example, 0.0060, preferably 0.0050. Whenthe birefringence is optimized to fall within such range, a λ/4 platethat is thin and has desired optical characteristics can be obtained.The Nz coefficient of the λ/4 plate is preferably from 0.9 to 3, morepreferably from 0.9 to 2.5, still more preferably from 0.9 to 1.5,particularly preferably from 0.9 to 1.3.

The λ/4 plate contains a resin having an absolute value of itsphotoelastic coefficient of preferably 2×10⁻¹¹m²/N or less, morepreferably from 2.0×10⁻¹³ m²/N to 1.6×10⁻¹¹ m²/N. When the absolutevalue of the photoelastic coefficient falls within such range, aretardation change is less liable to be generated in the case where ashrinkage stress is generated at the time of heating.

When the polarization converting layer is formed of the λ/4 plate, theoptical member preferably includes the low-refractive index layer asillustrated in FIG. 2 to FIG. 5.

In one embodiment, the λ/4 plate may include any appropriate resin filmthat can satisfy the characteristics. Typical examples of such resininclude a cyclic olefin-based resin, a polycarbonate-based resin, acellulose-based resin, a polyester-based resin, a polyvinylalcohol-based resin, a polyamide-based resin, a polyimide-based resin, apolyether-based resin, a polystyrene-based resin, and an acrylic resin.Any appropriate polycarbonate resin may be used as the polycarbonateresin as long as the effect of the present invention is obtained. Thepolycarbonate resin preferably contains: a structural unit derived froma fluorene-based dihydroxy compound; a structural unit derived from anisosorbide-based dihydroxy compound; and a structural unit derived fromat least one dihydroxy compound selected from the group consisting of analicyclic diol, an alicyclic dimethanol, di-, tri-, or polyethyleneglycol, and an alkylene glycol or spiroglycol. The polycarbonate resinmore preferably contains: a structural unit derived from afluorene-based dihydroxy compound; a structural unit derived from anisosorbide-based dihydroxy compound; and a structural unit derived froman alicyclic dimethanol and/or a structural unit derived from di-, tri-,or polyethylene glycol. The polycarbonate resin still more preferablycontains: a structural unit derived from a fluorene-based dihydroxycompound; a structural unit derived from an isosorbide-based dihydroxycompound; and a structural unit derived from di-, tri-, or polyethyleneglycol. The polycarbonate resin may contain a structural unit derivedfrom any other dihydroxy compound as required. Details of thepolycarbonate resin that may be suitably used in the present inventionare disclosed in, for example, Japanese Patent Application Laid-open No.2014-10291 and Japanese Patent Application Laid-open No. 2014-26266. Thedisclosures of the laid-open publications are incorporated herein byreference.

In another embodiment, the λ/4 plate may be an alignment fixed layer ofa liquid crystal compound. When the liquid crystal compound is used, adifference between nx and ny of the λ/4 plate to be obtained can beremarkably increased as compared to a non-liquid crystal material, andhence a thickness of the A/4 plate for obtaining a desired in-planeretardation can be remarkably reduced. Typically, a rod-shaped liquidcrystal compound is aligned in a state of being aligned in the slow axisdirection of the λ/4 plate (homogeneous alignment). An example of theliquid crystal compound is a liquid crystal compound whose liquidcrystal phase is a nematic phase (nematic liquid crystal). For example,a liquid crystal polymer or a liquid crystal monomer may be used as theliquid crystal compound. Any appropriate liquid crystal monomer may beadopted as the liquid crystal monomer. For example, a polymerizablemesogenic compound and the like disclosed in Japanese Patent TranslationPublication No. 2002-533742 (WO 00/37585 A1), European Patent No. 358208(U.S. Pat. No. 5,211,877), European Patent No. 66137 (U.S. Pat. No.4,388,453), NO 93/22397 A1, European Patent No. 0261712, German PatentNo. 19504224, German Patent No. 4408171, UK Patent No. 2280445, and thelike may be used. Specific examples of such polymerizable mesogeniccompound include a product available under the product name “LC242” fromBASF SE, a product available under the product name “E7” from MerckKGaA, and a product available under the product name“LC-Sillicon-CC3767” from Wacker Chemie AG. Specific examples of theliquid crystal compound and details of a method of forming the alignmentfixed layer are described in Japanese Patent Application Laid-open No.2006-163343. The description of the laid-open publication isincorporated herein by reference. In another embodiment, typically, adisc-shaped liquid crystal compound is aligned in any one of thefollowing states: vertical alignment, hybrid alignment, and tiltalignment. An example of the liquid crystal compound is a discoticliquid crystalline compound. The discotic liquid crystalline compound istypically such that the disc surface of the discotic liquid crystallinecompound is aligned substantially vertically with respect to the filmsurface of the λ/4 plate. For example, a discotic liquid crystallinecompound disclosed in Japanese Patent Application Laid-open No.2007-108732 or Japanese Patent Application Laid-open No. 2010-244038 maybe preferably used as the discotic liquid crystalline compound, but thediscotic liquid crystalline compound is not limited thereto.

E. Prism Sheet

The prism sheet 40 is arranged on the opposite side of the polarizationconverting layer 30 to the reflective polarizer 20. The prism sheet 40typically includes the substrate portion 41 and the prism portion 42. Asillustrated in FIG. 5, the substrate portion 41 is not necessarilyneeded to be arranged when the polarization converting layer 30 mayfunction as a substrate portion for supporting the prism portion 42.When the optical member of the present invention is arranged on thebacklight side of a liquid crystal display apparatus, the prism sheet 40guides polarized light, which has been output from the light guide plateof the backlight unit of the apparatus, as polarized light having themaximum intensity in an approximately normal direction of the liquidcrystal display apparatus to the polarizing plate 10 throughintermediation of the reflective polarizer 20 and the polarizationconverting layer 30 by means of, for example, total reflection in theprism portion 42 while maintaining the polarization state of the light.The term “approximately normal direction” comprehends a direction at apredetermined angle with respect to a normal direction, for example, adirection at an angle in the range of ±10° with respect to the normaldirection.

The prism sheet 40 is bonded to the adjacent layer throughintermediation of any appropriate adhesion layer (e.g., an adhesivelayer or a pressure-sensitive adhesive layer: not shown).

E-1. Prism Portion

In one embodiment, as illustrated in FIG. 1, the prism sheet 40(substantially the prism portion 42) includes an array of a plurality ofunit prisms 43, which are convex toward an opposite side to thereflective polarizer 20, in a parallel manner. Each of the unit prisms43 is preferably columnar, and its lengthwise direction (edge linedirection) is directed toward a direction approximately perpendicular,or a direction approximately parallel, to the transmission axis of thepolarizing plate 10 and the transmission axis of the reflectivepolarizer 20. In this specification, the expressions “substantiallyperpendicular” and “approximately perpendicular” include a case in whichan angle formed by two directions is 90°±10°, and the angle ispreferably 90°±7°, more preferably 90°±5°. The expressions“substantially parallel” and “approximately parallel” include a case inwhich an angle formed by two directions is 0°±10°, and the angle ispreferably 0°±7°, more preferably 0°±5°. Moreover, in thisspecification, such a simple expression “perpendicular” or “parallel”may include a substantially perpendicular state or a substantiallyparallel state. The prism sheet 40 may be arranged so that the edge linedirection of each of the unit prisms 43, and each of the transmissionaxis of the polarizing plate 10 and the transmission axis of thereflective polarizer 20 form a predetermined angle (the so-calledoblique arrangement). The adoption of such configuration can prevent theoccurrence of the moire in a more satisfactory manner in some cases. Therange of the oblique arrangement is preferably 20° or less, morepreferably 15° or less.

Any appropriate configuration may be adopted as the shape of each of theunit prisms 43 as long as the effect of the present invention isobtained. The shape of a section of each of the unit prisms 43 parallelto its arrangement direction and parallel to its thickness direction maybe a triangular shape or may be any other shape (e.g., such a shape thatone of, or each of both, the inclined planes of a triangle has aplurality of flat surfaces having different tilt angles). The triangularshape may be a shape asymmetric with respect to a straight line passingthe apex of the unit prism and perpendicular to the surface of the sheet(e.g., a scalene triangle), or may be a shape symmetric with respect tothe straight line (e.g., an isosceles triangle). Further, the apex ofthe unit prism may be of a chamfered curved surface shape, or may be ofa shape whose section is a trapezoid, the shape being obtained by suchcutting that its tip becomes a flat surface. Detailed shapes of the unitprisms 43 may be appropriately set depending on purposes. For example, aconfiguration disclosed in Japanese Patent Application Laid-open No. Hei11-84111 may be adopted for each of the unit prisms 43.

The distance between the prism portion 42 and the light diffusing layer50 is preferably from 75 μm to 250 μm. The securement of such distancebetween the prism portion and the light diffusing layer cansatisfactorily suppress the occurrence of the moire while maintainingthe front contrast and luminance of the liquid crystal displayapparatus. The distance between the prism portion 42 and the lightdiffusing layer 50 can be controlled by adjusting, for example, thethickness of the reflective polarizer 20, the substrate portion 41,and/or the adhesion layer between the reflective polarizer 20 and theprism sheet 40. The distance between the prism portion 42 and the lightdiffusing layer 50 refers to a distance between the flat surface of theprism portion 42 (surface opposite to the apices of the unit prisms 43)and the surface of the light diffusing layer 50 on the reflectivepolarizer 20 side.

E-2. Substrate Portion

When the substrate portion 41 is arranged in the prism sheet 40, thesubstrate portion 41 and the prism portion 42 may be integrally formedby, for example, subjecting a single material to extrusion, or the prismportion may be shaped on a film for the substrate portion. The thicknessof the substrate portion is preferably from 25 μm to 150 μm. Suchthickness is preferred from the viewpoints of the handling property andstrength of the prism sheet.

Any appropriate material may be adopted as a material for forming thesubstrate portion 41 depending on purposes and the configuration of theprism sheet. When the prism portion is shaped on the film for thesubstrate portion, the film for the substrate portion is specifically,for example, a film formed of cellulose triacetate (TAC), a (meth)acrylic resin, such as polymethyl methacrylate (PMMA), or apolycarbonate (PC) resin. The film is preferably an unstretched film.

When the substrate portion 41 and the prism portion 42 are integrallyformed of a single material, the same material as a material for formingthe prism portion when the prism portion is shaped on the film for thesubstrate portion can be used as the material. Examples of the materialfor forming the prism portion include epoxy acrylate- and urethaneacrylate-based reactive resins (e.g., an ionizing radiation-curableresin). When the prism sheet of an integral configuration is formed, apolyester resin, such as PC or PET, an acrylic resin, such as PISIA orMS, or an optically transparent thermoplastic resin, such as cyclicpolyolefin, can be used.

The photoelastic coefficient of the substrate portion 41 is preferablyfrom −10×10⁻¹² m²/N to 10×10⁻¹² m² more preferably from −5×10⁻¹² m²/N to5×10⁻¹² m²/N, still more preferably from −3×10⁻¹² m²/N to 3×10⁻¹² m²/N.

In one embodiment, the substrate portion 41 substantially has opticalisotropy. The phrase “substantially have optical isotropy” as usedherein means that a retardation value is so small as to havesubstantially no influences on the optical characteristics of the liquidcrystal display apparatus. For example, an in-plane retardation Re ofthe substrate portion is preferably 20 nm or less, more preferably 10 nmor less. In another embodiment, the polarization converting layer 30having the optical characteristics described in the section D mayfunction as the substrate portion.

F. Light Diffusing Layer

The light diffusing layer 50 may be arranged between the polarizingplate 10 and the reflective polarizer 20 as illustrated in FIG. 2, FIG.4, and FIG. 5, or may be arranged between the reflective polarizer 20and the prism sheet 40 as illustrated in FIG. 3. The light diffusinglayer 50 may be formed of a light diffusing element or may be formed ofa light diffusing pressure-sensitive adhesive. The light diffusingelement includes a matrix and light-diffusible fine particles dispersedin the matrix. The matrix of the light diffusing pressure-sensitiveadhesive is formed of a pressure-sensitive adhesive. The light diffusinglayer 50 is preferably integrated with the other constituent layers ofthe optical member.

The light diffusing performance of the light diffusing layer can berepresented by, for example, a haze value and/or a light diffusinghalf-value angle. The haze value of the light diffusing layer ispreferably from 10% to 99%, more preferably from 20% to 95%. The settingof the haze value within the range provides desired diffusingperformance and hence can satisfactorily suppress the occurrence of themoire. The light diffusing half-value angle of the light diffusing layeris preferably from 5° to 50°, more preferably from 10° to 30°. The lightdiffusing performance of the light diffusing layer can be controlled byadjusting, for example, a constituent material for the matrix (thepressure-sensitive adhesive in the case of the light diffusingpressure-sensitive adhesive), and a constituent material for, and thevolume-average particle diameter and compounding amount of, thelight-diffusible fine particles.

The total light transmittance of the light diffusing layer is preferably75% or more, more preferably 80% or more, still more preferably 85% ormore.

The thickness of the light diffusing layer may be appropriately adjusteddepending on, for example, its configuration and diffusing performance.For example, when the light diffusing layer is formed of the lightdiffusing element, the thickness is preferably from 5 μm to 200 μm. Inaddition, for example, when the light diffusing layer is formed of thelight diffusing pressure-sensitive adhesive, the thickness is preferablyfrom 5 μm to 100 μm.

When the light diffusing layer is formed of the light diffusing element,the matrix is formed of, for example, an ionizing radiation-curableresin. An ionizing radiation is, for example, UV light, visible light,an infrared ray, or an electron beam. Of those, the UV light ispreferred. Therefore, the matrix is preferably formed of a UV-curableresin. Examples of the UV-curable resin include an acrylic resin, analiphatic (e.g., polyolefin) resin, and a urethane-based resin.

The light diffusing layer is preferably formed of the light diffusingpressure-sensitive adhesive. The adoption of such configurationeliminates the need for an adhesion layer (an adhesive layer or apressure-sensitive adhesive layer) needed in the case where the lightdiffusing layer is formed of the light diffusing element. Accordingly,the adoption can contribute to the thinning of the optical member(consequently, a liquid crystal display apparatus) and eliminate theadverse effects of the adhesion layer on the display characteristics ofthe liquid crystal display apparatus. In this case, the light diffusinglayer contains a pressure-sensitive adhesive and light-diffusible fineparticles dispersed in the pressure-sensitive adhesive.

Any appropriate pressure-sensitive adhesive and light-diffusible fineparticles may be used as the pressure-sensitive adhesive and thelight-diffusible fine particles. Details of the pressure-sensitiveadhesive and the light-diffusible fine particles are described in, forexample, Japanese Patent Application Laid-open No. 2014-224964. Theentire description of the laid-open publication is incorporated hereinby reference.

The content of the pressure-sensitive adhesive in the light diffusingpressure-sensitive adhesive is preferably from 50 wt % to 99.7 wt %,more preferably from 52 wt % to 97 wt %.

The content of the light-diffusible fine particles in the lightdiffusing pressure-sensitive adhesive is preferably from 0.3 wt % to 50wt %, more preferably from 3 wt % to 48 wt %. When the content of thelight-diffusible fine particles is set to fall within theabove-mentioned range, a light diffusing pressure-sensitive adhesivelayer having excellent light diffusing performance can be obtained.

The pressure-sensitive adhesive may contain any appropriate additiveand/or cross-linking agent. Examples of the additive include anantistatic agent, an antioxidant, and a coupling agent. The kinds,addition amounts, combination, and the like of the additives may beappropriately set depending on purposes. Examples of the cross-linkingagent include an organic cross-linking agent and a polyfunctional metalchelate.

G. Low-Refractive Index Layer

The low-refractive index layer 60 may be arranged between the polarizingplate 10 and the reflective polarizer 20 as illustrated in FIG. 2 andFIG. 3, or may be arranged between the reflective polarizer 20 and theprism sheet 40 as illustrated in FIG. 4 and FIG. 5. The low-refractiveindex layer 60 is preferably integrated with the other constituentlayers of the optical member.

The thickness of the low-refractive index layer is preferably from 0.2μm to 5 μm, more preferably from 0.3 μm to 3 μm. The refractive index ofthe low-refractive index layer 60 is preferably 1.30 or less, morepreferably 1.20 or less. The lower limit of the refractive index of thelow-refractive index layer is, for example, 1.01. When the refractiveindex of the low-refractive index layer falls within such range, theutilization efficiency of light can be enhanced by sufficiently securingthe amount of light to be output in a front direction and suppressingthe generation of light in a direction in which the light cannot beoutput to a viewer side. As a result, a liquid crystal display apparatushaving a high luminance can be achieved through the use of the opticalmember.

The low-refractive index layer 60 typically has a void in itself. Thevoid ratio of the low-refractive index layer is, for example, from 5% to90%, preferably from 25% to 80%. When the void ratio falls within therange, the low-refractive index layer can be sufficiently reduced inrefractive index, and a high mechanical strength can be achieved.

The low-refractive index layer having a void in itself is, for example,a low-refractive index layer at least partially having a porous layerand/or an air layer. The porous layer typically contains aerogel and/orparticles (e.g., hollow fine particles and/or porous particles). Thelow-refractive index layer may be preferably a nanoporous layer(specifically a porous layer 90% or more of the fine pores of which eachhave a diameter in the range of from 10⁻¹ to 10³ nm).

Any appropriate material may be adopted as a material for forming thelow-refractive index layer. For example, materials disclosed in WO2004/113966 A1, Japanese Patent Application Laid-open No. 2013-254183,and Japanese Patent Application Laid-open No. 2012-189802 may each beadopted as the material. Specific examples thereof include: silica-basedcompounds; hydrolyzable silanes, and partial hydrolysates anddehydration condensates thereof; organic polymers; silicon compoundseach containing a silanol group; active silica obtained by bringing asilicate into contact with an acid or an ion exchange resin;polymerizable monomers (e.g., a (meth)acrylic monomer and astyrene-based monomer); curable resins (e.g., a (meth) acrylic resin, afluorine-containing resin, and a urethane resin); and a combinationthereof.

Any appropriate particles may be adopted as the particles. The particlesare each typically formed of a silica-based compound. Examples of theshape of each of the particles include a spherical shape, a plate-likeshape, a needle-like shape, a string-like shape, and a grapecluster-like shape. Examples of a particle of the string-like shapeinclude: a particle obtained by connecting a plurality of particles eachhaving a spherical shape, a plate-like shape, or a needle-like shape ina beaded manner; a short fibrous particle (e.g., a short fibrousparticle disclosed in Japanese Patent Application Laid-open No.2001-188104); and a combination thereof. The string-shaped particle maybe linear or may be branched. A particle of the grape cluster-like shapeis, for example, a particle of a grape cluster-like shape obtained bythe agglomeration of a plurality of particles having spherical,plate-like, and needle-like shapes. The shapes of the particles may beconfirmed by, for example, observation with a transmission electronmicroscope. The average particle diameter of the particles is, forexample, from 5 nm to 200 nm, preferably from 10 nm to 200 nm. Thepresence of the above-mentioned configuration can provide alow-refractive index layer having a sufficiently low refractive indexand can maintain the transparency of the low-refractive index layer. Theterm “average particle diameter” as used herein means a value determinedby using a specific surface area (m²/g) measured by a nitrogenadsorption method (BET method) from an equation “average particlediameter=(2,720/specific surface area)” (see Japanese Patent ApplicationLaid-open No. Hei 1-317115).

Examples of a method of obtaining the low-refractive index layer includemethods disclosed in Japanese Patent Application Laid-open No.2010-189212, Japanese Patent Application Laid-open No. 2008-040171,Japanese Patent Application Laid-open No. 2006-011175, WO 2004/113966A1, and references thereof. Specific examples thereof include: a methodinvolving subjecting at least one of silica-based compounds, andhydrolyzable silanes, and partial hydrolysates and dehydrationcondensates thereof to hydrolysis and polycondensation; a methodinvolving using porous particles and/or hollow fine particles; and amethod involving utilizing a spring-back phenomenon to produce anaerogel layer.

H. Polarizing Plate Set

The optical member of the present invention can be typically used as apolarizing plate arranged on the opposite side of a liquid crystaldisplay apparatus to its viewer side (hereinafter sometimes referred toas “back-surface side polarizing plate”). In this case, a polarizingplate set including the back-surface side polarizing plate and a viewerside polarizing plate can be provided. Any appropriate polarizing platemay be adopted as the viewer side polarizing plate. The viewer sidepolarizing plate typically includes a polarizer (e.g., anabsorption-type polarizer) and a protective layer arranged on at leastone side of the polarizer. Those described in the section B can be usedas the polarizer and the protective layer. The viewer side polarizingplate may further have any appropriate optical functional layer (e.g., aretardation layer, a hard coat layer, an antiglare layer, or anantireflection layer) depending on purposes. The polarizing plate set isarranged on each side of a liquid crystal cell so that the absorptionaxis of (the polarizer of) the viewer side polarizing plate and theabsorption axis of (the polarizer of) the back-surface side polarizingplate are substantially perpendicular or parallel to each other.

I. Liquid Crystal Display Apparatus

FIG. 7 is a schematic sectional view of a liquid crystal displayapparatus according to one embodiment of the present invention. A liquidcrystal display apparatus 500 includes a liquid crystal cell 200, aviewer side polarizing plate 110 arranged on the viewer side of theliquid crystal cell 200, the Optical member 100 of the present inventionserving as a back-surface side polarizing plate arranged on the oppositeside of the liquid crystal cell 200 to the viewer side, and a backlightunit 300 arranged on the opposite side of the optical member 100 to theliquid crystal cell 200. In the liquid crystal display apparatus 500,each of the optical members 101 to 104 may be used instead of theoptical member 100. The viewer side polarizing plate is as described inthe section H. In the illustrated example, the viewer side polarizingplate 110 includes the polarizer 11, the protective layer 12 arranged onone side of the polarizer, and the protective layer 13 arranged on theother side of the polarizer 11. The viewer side polarizing plate 110 andthe optical member (back-surface side polarizing plate) 100 are arrangedso that their respective absorption axes are substantially perpendicularor parallel to each other. Any appropriate configuration may be adoptedfor the backlight unit 300. For example, the backlight unit 300 may beof an edge light system or may be of a direct system. When the directsystem is adopted, the backlight unit 300 includes, for example, a lightsource, a reflective film, and a diffuser (none of which is shown) Whenthe edge light system is adopted, the backlight unit 300 can furtherinclude a light guide plate and a light reflector (none of which isshown).

The liquid crystal cell 200 includes a pair of substrates 210 and 210′,and a liquid crystal layer 220 serving as a display medium sandwichedbetween the substrates. in a general configuration, on the substrate210′ serving as one in the pair, a color filter and a black matrix arearranged, and on the substrate 210 serving as the other in the pair,there are arranged switching elements for controlling theelectro-optical property of the liquid crystal, scanning lines forgiving gate signals to the switching elements and signal lines forgiving source signals thereto, and pixel electrodes and counterelectrodes. An interval (cell gap) between the above-mentionedsubstrates 210 and 210′ can be controlled by spacers and the like. Onsides of the above-mentioned substrates 210 and 210, which are broughtinto contact with the liquid crystal layer 220, for example, alignmentfilms made of polyimide and the like can be arranged.

Now, the present invention is specifically described by way of Examples.However, the present invention is not limited by these Examples.

EXAMPLE 1 1. Production of Optical Member 1-1. Production of PolarizingPlate

A polyvinyl alcohol film having a thickness of 80 μm was stretched to 3times between rolls having different speed ratios while being dyed in aniodine solution at 30° C. having a concentration of 0.3% for 1 minute.After that, the film was stretched until the total stretching ratiobecame 6 times while being immersed in an aqueous solution at 60° C.containing boric acid at a concentration of 4% and potassium iodide at aconcentration of 10% for 0.5 minute. Subsequently, the film was washedby being immersed in an aqueous solution at 30° C. containing potassiumiodide at a concentration of 1.5% for 10 seconds, and was then dried at50° C. for 4 minutes to provide a polarizer. To each of both surfaces ofthe polarizer, a saponified triacetylcellulose film having a thicknessof 80 μm was bonded through the use of a polyvinyl alcohol-basedadhesive. Thus, a polarizing plate was produced.

1-2. Production of Polarization Converting Layer

A high-retardation layer was used as a polarization converting layer,and a biaxially stretched PET film (manufactured by Toyobo Co., Ltd.,product name: “A4300”, thickness: 75 μm) was used as thehigh-retardation layer. The in-plane retardation Re of the biaxiallystretched PET film was 4,000 nm.

1-3. Production of Reflective Polarizer

A 40-inch TV manufactured by Sharp Corporation (product name: AQUOS,item's stock number: LC40-Z5) was dismantled, and a reflective polarizerwas removed from its backlight member. Diffusing layers arranged on bothsurfaces of the reflective polarizer were removed, and the remainder wasdefined as a reflective polarizer (thickness: 92 μm) of this Example.

1-4. Production of Prism Sheet

An acrylic resin film (thickness; 40 μm) obtained by a production methoddescribed in Production Example 1 of Japanese Patent ApplicationLaid-open No. 2012-234163 was used as a film for a substrate portion. Apredetermined mold having placed therein the acrylic resin film wasfilled with a UV-curable urethane acrylate resin serving as a materialfor a prism, and the material for a prism was cured by being irradiatedwith UV light. Thus, a prism sheet as illustrated in FIG. 1 wasproduced. The in-plane retardation Re of its substrate portion was 0.4nm. Its unit prism was a triangular prism, and the shape of across-section of the unit prism parallel to an arrangement direction andparallel to a thickness direction was a scalene triangle shape.

1-5. Production of Optical Member

The biaxially stretched PET film, the reflective polarizer, and thepolarizing plate were bonded in the stated order to the substrateportion side of the prism sheet through intermediation of apressure-sensitive adhesive to produce an optical member. The componentswere arranged so that: the absorption axis of the polarizing plate andthe reflection axis of the reflective polarizer were parallel to eachother; an angle formed between the reflection axis of the reflectivepolarizer and the slow axis of the biaxially stretched PET film was 45°;and the absorption axis of the polarizing plate and the edge line of theprism sheet were parallel to each other.

2. Production of Optical Member Including No Reflective Polarizer

The biaxially stretched PET film and the polarizing plate were bonded inthe stated order to the substrate portion side of the prism sheetthrough intermediation of a pressure-sensitive adhesive to produce anoptical member including no reflective polarizer. The components werearranged so that: an angle formed between the absorption axis of thepolarizing plate and the slow axis of the biaxially stretched PET filmwas 45°; and the absorption axis of the polarizing plate and the edgeline of the prism sheet were parallel to each other.

EXAMPLE 2

An optical member and an optical member including no reflectivepolarizer Were produced in the same manner as in Example 1 except that:a light diffusing layer was used as the polarization converting layer;and a light diffusing sheet removed from a 40-inch TV manufactured bySharp Corporation (product name: AQUOS, item's stock number: L 40-Z5)through its dismantlement was used as the light diffusing layer. Thehaze value of the light diffusing sheet was 92%.

EXAMPLE 3 1. Production of Polarization Converting layer

A λ/4 plate was used as a polarization converting layer, and a λ/4 plateobtained by obliquely stretching a cycloolefin-based resin film(manufactured by Zeon Corporation, “ZEONOR ZF-14 Film”) at a stretchingangle of 45° with a tenter stretching machine was used as the λ/4 plate.The in-plane retardation Re (550) of the obtained λ/4 plate was 90 nm.

2. Production of Acrylic Film with Low-refractive Index layer

(1) Gelation of Silicon Compound

0.95 g of MTMS serving as a precursor of a silicon compound wasdissolved in 2.2 g of DMSO. To the thus obtained mixed liquid, 0.5 g ofa 0.01 mol/L oxalic acid aqueous solution was added, and the mixture wasstirred at room temperature for 30 minutes to hydrolyze the MTMS. Thus,tris (hydroxy)methylsilane was produced.

To 5.5 g of DMSO, 0.38 g of ammonia water having a concentration of 28%,and 0.2 g of pure water were added, and then the mixed liquid subjectedto the hydrolysis treatment was further added. The mixture was stirredat room temperature for 15 minutes to perform gelation of the tris(hydroxy)methylsilane. Thus, a gelled silicon compound was obtained.

(2) Maturation Treatment

The mixed liquid subjected to the gelation treatment was subjected tomaturation treatment by being incubated as it was at 40° C. for 20hours.

(3) Pulverization Treatment

Next, the gelled silicon compound subjected to the maturation treatmentwas crushed with a spatula into a granular shape having a size of fromseveral millimeters to several centimeters. To the crushed product, 40 gof IPA was added, and the mixture was lightly stirred. After that, themixture was left to stand still at room temperature for 6 hours, and thesolvent and catalyst in the gel were decanted. The same decantationtreatment was repeated 3 times to complete solvent replacement. Then,the gelled silicon compound in the mixed liquid was subjected topulverization treatment (high-pressure media-less pulverization). Thepulverization treatment (high-pressure media-less pulverization) wasperformed by weighing out 1.85 g of the gelled compound after thecompletion of the solvent replacement and 1.15 g of IPA in a 5 cm³screw-capped vial, and then pulverizing the contents under theconditions of 50 W and 20 kHz for 2 minutes through the use of ahomogenizer (product name: UH-50, manufactured by SMT Corporation).

As a result of the pulverization of the gelled silicon compound in themixed liquid through the pulverization treatment, the mixed liquidbecame a sol liquid of the pulverized product. A volume-average particlediameter indicating the particle size variation of the pulverizedproduct contained in the mixed liquid was found to be from 0.50 to 0.70with a dynamic light scattering Nanotrac particle size analyzer(manufactured by Nikkiso Co., Ltd., model UPA-EX150). Further, to 0.75 gof the sol liquid, a solution of a photobase generator (Wako PureChemical industries, Ltd.: product name: WPBG-266) in methyl ethylketone (MEK) at a concentration of 1.5% and a solution of bis(trimetboxysilyl) ethane in MEK at a concentration of 5% were added at aratio of 0.062 g: 0.036 g to provide an application liquid.

(4) Formation of Low-refractive Index Layer

The application liquid was applied to the substrate surface of anacrylic resin film (thickness: 40 μm) prepared in accordance withProduction Example 1 of Japanese Patent Application Laid-open No.2012-234163 to form an applied film. The applied film was dried by beingtreated at a temperature of 100° C. for 1 minute, and further, theapplied film after the drying was subjected to UV irradiation with lighthaving a wavelength of 360 nm at a light irradiation dose (energy) of300 mJ/cm² to provide a laminate having a low-refractive index layerformed on the acrylic resin film (acrylic film with a low-refractiveindex layer). The refractive index of the low-refractive index layer was1.18.

3. Production of Optical Member and Optical Member Including NoReflective Polarizer

An optical member was produced in the same manner as in Example 1 exceptthat: the reflective polarizer and the λ/4 plate were arranged so thatthe angle formed between the reflection axis of the reflective polarizerand the slow axis of the λ/4 plate was 45°; and the acrylic film with alow-refractive index layer was arranged between the polarizing plate andthe reflective polarizer. In addition, an optical member including noreflective polarizer was produced in the same manner as in Example 1except that: the polarizing plate and the λ/4 plate were arranged sothat the angle formed between the absorption axis of the polarizingplate and the slow axis of the λ/4 plate was 45°; and the acrylic filmwith a low-refractive index layer was arranged between the polarizingplate and the λ/4 plate.

Comparative Example 1

An optical member and an optical member including no reflectivepolarizer were produced in the same manner as in Example 1 except thatno polarization converting layer was used.

Comparative Example 2

An optical member was produced in the same manner as in Example 3 exceptthat the reflective polarizer and the λ/4 plate were arranged so thatthe angle formed between the reflection axis of the reflective polarizerand the slow axis of the λ/4 plate was 0°. In addition, an opticalmember including no reflective polarizer was produced in the same manneras in Example 3 except that the polarizing plate and the λ/4 plate werearranged so that the angle formed between the absorption axis of thepolarizing plate and the slow axis of the λ/4 plate was 0°.

(Evaluation)

Characteristics of the optical member of each of Examples andComparative Examples were measured as described below.

1. Measurement of Polarization Conversion Degree P_(CON) 1-1. PolarizingPlate for Evaluation

In accordance with the following procedure, a first polarizing plate andsecond polarizing plate to be used for the measurement of a polarizationconversion degree P_(CON) were produced. First, a polyvinyl alcohol filmhaving a thickness of 80 μm was stretched to 3 times between rollshaving different speed ratios while being dyed in an iodine solution at30° C. having a concentration of 0.3% for 1 minute. After that, the filmwas stretched until the total stretching ratio became 6 times whilebeing immersed in an aqueous solution at 60° C. containing boric acid ata concentration of 4% and potassium iodide at a concentration of 10% for0.5 minute. Subsequently, the film was washed by being immersed in anaqueous solution at 30° C. containing potassium iodide at aconcentration of 1.5% for 10 seconds, and was then dried at 50° C. for 4minutes to provide a polarizer. To each of both surfaces of thepolarizer, a saponified triacetylcellulose film having a thickness of 80μm was bonded through the use of a polyvinyl alcohol-based adhesive toproduce the first polarizing plate. The second polarizing plate wassimilarly produced.

The obtained first polarizing plate and second polarizing plate wereeach measured for a single layer transmittance (Ts), a paralleltransmittance (Tp), and a cross transmittance (Tc) with a UV-visiblespectrophotometer (V-7100 manufactured by JASCO Corporation), and apolarization degree (P) was determined by the following equation. Thosetransmittances are Y values measured with the two-degree field of view(C light source) of JIS Z 8701 and subjected to visibility correction.Light absorption of the triacetylcellulose film serving as a protectivelayer is negligibly small as compared to light absorption of thepolarizer, and hence the transmittances of each polarizing plate weredefined as the transmittances of its polarizer.

Polarization degree (P)={(Tp−Tc)/(Tp+Tc)}^(1/2)×100

The polarization degree of the polarizer of each of the first polarizingplate and the second polarizing plate was 99.99%.

1-2. Polarization Conversion Degree

A backlight light source removed from a liquid crystal display apparatus(manufactured by Sony Corporation, product name: “VAIO-S”), the firstpolarizing plate, the polarization converting layer used in each ofExamples and Comparative Examples, the second polarizing plate, and aconoscope (manufactured by Autronic-Melchers GmbH) were arranged in thestated order, and the luminance of the light source was measured (unit:cd/m²) through intermediation of the first polarizing plate, thepolarization converting layer, and the second polarizing plate. Theluminance when the respective polarizers of the first polarizing plateand the second polarizing plate were arranged so that the absorptionaxes thereof were parallel to each other was defined as a parallelluminance L₀, and the luminance when the respective polarizers of thefirst polarizing plate and the second polarizing plate were arranged sothat the absorption axes thereof were perpendicular to each other wasdefined as a perpendicular luminance L₉₀. The polarization conversiondegree P_(CON) exhibited by the polarization converting layer wasdetermined by the following equation.

Polarization conversion degree (P_(CON))=L₉₀/L₀

2. Measurement of Luminance Enhancement Degree Exhibited by ReflectivePolarizer

A glass plate having a thickness of 0.4 mm was bonded to the polarizingplate side of an optical member through intermediation of apressure-sensitive adhesive to produce a measurement sample. A glassplate having a thickness of 0.4 mm was bonded to the polarizing plateside of an optical member including no reflective polarizer throughintermediation of a pressure-sensitive adhesive to produce a measurementsample for comparison. A backlight light source removed from a liquidcrystal display apparatus (manufactured by Sony Corporation, productname: “VAIO-S”) was arranged on the back-surface side of the measurementsample (opposite side to the glass plate), and the front luminance oflight transmitted through the measurement sample was measured (unit:cd/m²) with a conoscope (manufactured by Autronic-Melchers GmbH).Similarly, the front luminance of light transmitted through themeasurement sample for comparison was measured (unit: cd/m²). Aluminance enhancement degree C (%) exhibited by the reflective polarizerwas determined by the following equation on the basis of the frontluminance A of the light transmitted through the measurement sample andthe front luminance B of the light transmitted through the measurementsample for comparison.

Luminance enhancement degree (C)=((A/B)−1)×100 (%)

3. Measurement of In-plane Retardations of High-retardation Layer andλ/4 Plate

A sample having a size of 50 mm×50 mm was cut out of each of thehigh-retardation layer and the λ/4 plate used in Examples 1 and 3, andthe λ/4 plate used in Comparative Example 2, and was used as ameasurement sample. The produced measurement sample was measured for anin-plane retardation with Axoscan (manufactured by Axometrics, Inc.). Ameasurement temperature was set to 23° C., and a measurement wavelengthwas set to 550 nm.

4. Measurement of Haze Value

The haze value of the light diffusing layer used in Example 2 wasmeasured by a method specified in JIS 7136 with a haze meter(manufactured by Murakami Color Research Laboratory Co., Ltd., productname: “HN-150”).

5. Measurement of Refractive Index of Low-refractive Index layer

The low-refractive index layer used in Example 3 was measured for arefractive index using an Abbe refractometer (DR-M2, manufactured byAtago Co., Ltd.).

The luminance enhancement degree (C) of the optical member of each ofExamples and Comparative Examples, and the polarization conversiondegree (P_(CON)) of the polarization converting layer of each opticalmember are shown in Table 1.

TABLE 1 Polarization Polarization Luminance converting conversionenhancement layer degree degree Example 1 High-retardation 0.56 22%layer Example 2 Light diffusing 6.2 25% layer Example 3 λ/4 plate (45°)31.6 29% Comparative — 0.06  6% Example 1 Comparative λ/4 plate (0°) 0.19 11% Example 2

As apparent from Table 1, as the polarization conversion degree(P_(CON)) of the polarization converting layer increased, the luminanceenhancement degree (C) of the optical member increased, i.e., a higherluminance enhancement effect of the use of the reflective polarizer wasobtained.

The optical member of the present invention can be suitably used as theback-surface side polarizing plate of a liquid crystal displayapparatus. The liquid crystal display apparatus using such opticalmember can be used for various applications, such as portable devicesincluding a personal digital assistant (PDA), a cellular phone, a watch,a digital camera, and a portable gaming Machine, OA devices including apersonal computer monitor, a notebook-type personal computer, and acopying machine, electric home appliances including a video camera, aliquid crystal television set, and a microwave oven, on-board devicesincluding a reverse monitor, a monitor for a car navigation system, anda car audio, exhibition devices including an information monitor for acommercial store, security devices including a surveillance monitor, andcaring/medical devices including a caring monitor and a medical monitor.

What is claimed is:
 1. An optical member, comprising: a polarizingplate; a reflective polarizer; a polarization converting layer; and aprism layer, the polarizing plate, the reflective polarizer, thepolarization converting layer, and the prism layer being integrated inthe stated order, wherein the polarization converting layer satisfiesthe following expression:L₉₀/L₀≧0.2 where, with regard to a luminance of natural lighttransmitted through a first polarizer and a second polarizer each havinga polarization degree of 99.99%, L₀ represents a parallel luminance,which is the luminance when the polarization converting layer isarranged between the first polarizer and the second polarizer arrangedso that absorption axes thereof are parallel to each other, and L90represents a perpendicular luminance, which is the luminance when thepolarization converting layer is arranged between the first polarizerand the second polarizer arranged so that the absorption axes thereofare perpendicular to each other.
 2. The optical member according toclaim 1, wherein the polarization converting layer comprises aretardation layer, and wherein the retardation layer has an in-planeretardation Re(550) of 3,500 nm or more, where Re(550) represents anin-plane retardation measured with light having a wavelength of 550 nmat 23° C.
 3. The optical member according to claim 1, wherein thepolarization converting layer comprises a light diffusing layer, andwherein the light diffusing layer has a haze value of from 80% to 99.9%.4. The optical member according to claim 1, wherein the polarizationconverting layer comprises a λ/4 plate, wherein the λ/4 plate has anin-plane retardation Re(550) of from 80 nm to 200 nm, and wherein anangle formed between a slow axis of the λ/4 plate and a reflection axisof the reflective polarizer is from 30° to 60°.
 5. The optical memberaccording to claim 1, further comprising a low-refractive index layerintegrated at one of a position between the reflective polarizer and theprism layer, and a position on an opposite side of the reflectivepolarizer to the prism layer, wherein the low-refractive index layer hasa refractive index of 1.30 or less.
 6. The optical member according toclaim 1, wherein the reflective polarizer comprises a linearly polarizedlight separation-type reflective polarizer.